The architecture of a computer system typically comprises a bus structure consisting of a plurality of transmission lines to which various units are connected in parallel. In a computer system which includes a large number of units, as in a multi-processor system for example, the physical length of the bus becomes rather large. A disadvantage of this structure consists in the fact that the length of the bus increases the signal propagation time; this reduces the operating frequency of the bus, since the duration of an operating cycle is inevitably greater than this propagation time. Furthermore, since the same data item is distributed simultaneously to all the units connected to the bus, the structure is affected by the electrical load (input impedance) introduced by these units; this makes it necessary to use driver circuits with relatively high power and consequently high consumption, and creates a non-uniform distribution of the electrical load which may give rise to phenomena of reflection. The bus therefore has a low transfer rate, which has a marked effect on the performance of the whole computer system.
A further disadvantage is manifested in the case in which the bus (known as the remote bus) is used to connect nodes which comprise different units interconnected by means of a further bus (called the local bus).
The nodes are connected to the system or remote bus by a device which acts as a bridge between the local bus and the remote bus.
Each node corresponds to a single load (that of the bridge) connected to the remote bus.
In this way it is possible to reduce the number of loads connected to the bus and to improve its performance.
However, the remote bus generally has a greater length than the local buses, and therefore its operating speed is lower; this means that whenever a node accesses the remote bus it is necessary to introduce a latency period of a few operating cycles of the local bus.